Prestress losses in reinforced concrete beams strengthened with NSM-CFRP laminates

Using prestressed near surface mounted fibre reinforced polymers (NSM-FRP) is nowadays regaining the attention from the scientific community for the strengthening of existing reinforced concrete (RC) structures. The application of prestressed internal FRP bars and externally bonded prestressed FRPs has already been deeply investigated and revealed considerable benefits when compared to the corresponding passive solutions. A certain amount of prestress provides benefits mainly associated to structural integrity and material durability. Immediately after prestress transference, it is possible to close some of the existing cracks, decreasing the susceptibility of the element to corrosion and, a certain amount of deflection can be recovered due to the creation of a negative curvature. However, very few studies have been carried out to properly assess the preservation of prestress over time. In this context, several reinforced concrete beams strengthened with prestressed NSM carbon FRP (CFRP) laminates were prestressed and monitored for about 40 days. The data obtained from these experimental programs is in this paper presented and analysed. The observed prestress losses were later modelled using finite elements analysis and, although this topic is not addressed in this paper, the obtained results revealed considerable precision. The largest strain losses in the CFRP laminate were found to be mainly located in the extremities of the bonded length, while in the central zone most of the applied pre-strain was retained over time. The highest CFRP strain losses were observed in the first 6 to 12 days after prestress transfer, suggesting that the application of prestressed NSM-FRP will be very effective over time.

Numerical approaches for the analysis of timber frame walls

A numerical approach to simulate the behaviour of timber shear walls under both static and dynamic loading is proposed. Because the behaviour of timber shear walls hinges on the behaviour of the nail connections, the force-displacement behaviour of sheathing-to-framing nail connections are first determined and then used to define the hysteretic properties of finite elements representing these connections. The model nails are subsequently implemented into model walls. The model walls are verified using experimental results for both monotonic and cyclic loading. It is demonstrated that the complex hysteretic behaviour of timber shear walls can be reasonably represented using model shear walls in which nonlinear material failure is concentrated only at the sheathing-to-framing nail connections.

Deteção de colisões com guardas de segurança

Dissertação de mestrado integrado em Engenharia Eletrónica Industrial e Computadores

Estudo numérico sobre o reforço de pilares de betão armado com cantoneiras e presilhas metálicas: análise paramétrica e modelo de cálculo

Dissertação de mestrado integrado em Engenharia Civil

Implementation of a probabilistic based framework for safety assessment of existing timber elements

The assessment of existing timber structures is often limited to information obtained from non or semi destructive testing, as mechanical testing is in many cases not possible due to its destructive nature. Therefore, the available data provides only an indirect measurement of the reference mechanical properties of timber elements, often obtained through empirical based correlations. Moreover, the data must result from the combination of different tests, as to provide a reliable source of information for a structural analysis. Even if general guidelines are available for each typology of testing, there is still a need for a global methodology allowing to combine information from different sources and infer upon that information in a decision process. In this scope, the present work presents the implementation of a probabilistic based framework for safety assessment of existing timber elements. This methodology combines information gathered in different scales and follows a probabilistic framework allowing for the structural assessment of existing timber elements with possibility of inference and updating of its mechanical properties, through Bayesian methods. The probabilistic based framework is based in four main steps: (i) scale of information; (ii) measurement data; (iii) probability assignment; and (iv) structural analysis. In this work, the proposed methodology is implemented in a case study. Data was obtained through a multi-scale experimental campaign made to old chestnut timber beams accounting correlations of non and semi-destructive tests with mechanical properties. Finally, different inference scenarios are discussed aiming at the characterization of the safety level of the elements.

Numerical simulation of galvanized rebars pullout

The usage of rebars in construction is the most common method for reinforcing plain concrete and thus bridging the tensile stresses along the concrete crack surfaces. Usually design codes for modelling the bond behaviour of rebars and concrete suggest a local bond stress – slip relationship that comprises distinct reinforcement mechanisms, such as adhesion, friction and mechanical anchorage. In this work, numerical simulations of pullout tests were performed using the finite element method framework. The interaction between rebar and concrete was modelled using cohesive elements. Distinct local bond laws were used and compared with ones proposed by the Model Code 2010. Finally an attempt was made to model the geometry of the rebar ribs in conjunction with a material damaged plasticity model for concrete.

Exploring the use of HPFRC and GFRP grids for the production of manhole covers

Corrosion of the steel reinforced concrete elements is one of the common pathologies that limits the long-term performance of urban infrastructures. This problem causes the loss of structural serviceability by decreasing the concrete-steel bond strength and reducing the cross section of the reinforcements. The present study introduces a new system for developing free-corrosion resistance prefabricated manhole covers for applications in the aggressive environments, i.e. wastewater collector systems, sewer systems, stormwater systems, etc. Fibre reinforced cement composites were applied in this system in order to suppress the corrodible steel mesh and maintain the structural ductility as well. Application of fibre reinforced polymer (FRP) system is adopted as the additional solution for increasing the load carrying capacity of these elements without concerns about corrosion. The effectiveness of the applied strategy in developing the manhole covers in terms of load carrying capacity and failure mode is evaluated in this research. Furthermore, this paper discusses a FEM-based simulation, aiming to address the possibility of calibrating the constitutive model parameters related to fracture modes I and II.

Thin panels of cement composites reinforced with recycled fibres for the shear strengthening of reinforced concrete elements

A new technique was developed for producing thin panels of a cement based material reinforced with relatively high content of steel fibres originated from the industry of tyre recycling. Flexural tests with notched and un-notched specimens were carried out to characterize the mechanical properties of this Fibre Reinforced Cement Composite (FRCC) and the results are presented and discussed. The values of the fracture mode I parameters of the developed FRCC were determined by performing inverse analysis with test results obtained in three point notched beam bending tests. To appraise the potentialities of these FRCC panels for the increase of the shear capacity of reinforced (RC) beams, numerical research was performed on the use of developed FRCC panel for shear reinforcement by applying the panels in the lateral faces of RC beams deficiently reinforced in shear.

Exploring the potentialities of a new type of CFRP laminate for the simultaneous flexural and shear strengthening of RC beams: numerical research

This study aims to develop an innovative carbon fibre reinforced polymer (CFRP) laminate with a U configuration to address strengthening interventions, where the increment of both flexural and shear capacity of reinforced concrete (RC) elements is required. This strengthening solution combines the near surface mounted (NSM) and embedded through section (ETS) techniques in the same application, since these techniques have already evidenced high performance on flexural and shear strengthening of RC beams using FRP systems, respectively. In fact, the proposed hybrid technique aims to mobilize the advantages provided by these two strengthening techniques by using an innovative CFRP laminate. The strengthening efficacy of this new hybrid NSM/ETS technique was numerically assessed and compared to the corresponding efficiency of NSM and ETS techniques applied separately for the flexural and shear strengthening of RC beams, respectively. The numerical models are described and the main relevant results are presented and discussed.

High performance fiber reinforced concrete for the shear reinforcement: experimental and numerical research

High performance fiber reinforced concrete (HPFRC) is developing rapidly to a modern structural material with unique rheological and mechanical characteristics. Despite applying several methodologies to achieve self15 compacting requirements, some doubts still remain regarding the most convenient strategy for developing a HPFRC. In the present study, an innovative mix design method is proposed for the development of high17 performance concrete reinforced with a relatively high dosage of steel fibers. The material properties of the developed concrete are assessed, and the concrete structural behavior is characterized under compressive, flexural and shear loading. This study better clarifies the significant contribution of fibers for shear resistance of concrete elements. This paper further discusses a FEM-based simulation, aiming to address the possibility of calibrating the constitutive model parameters related to fracture modes I and II.

Recycled steel fibre reinforced concrete failing in bending and in shear

Recent research is showing that the addition of Recycled Steel Fibres (RSF) from wasted tyres can decrease significantly the brittle behaviour of cement based materials, by improving its toughness and post-cracking resistance. In this sense, Recycled Steel Fibre Reinforced Concrete (RSFRC) seems to have the potential to constitute a sustainable material for structural and non-structural applications. To assess this potential, experimental and numerical research was performed on the use of RSFRC in elements failing in bending and in beams failing in shear. The values of the fracture mode I parameters of the developed RSFRC were determined by performing inverse analysis with test results obtained in three point notched beam bending tests. To assess the possibility of using RSF as shear reinforcement in Reinforced Concrete (RC) beams, three point bending tests were executed with three series of RSFRC beams flexurally reinforced with a relatively high reinforcement ratio of longitudinal steel bars in order to assure shear failure for all the tested beams. By performing material nonlinear simulations with a computer program based on the finite element method (FEM), the applicability of the fracture mode I crack constitutive law derived from the inverse analysis is assessed for the prediction of the behaviour of these beams. The performance of the formulation proposed by RILEM TC 162 TDF and CEB-FIP 2010 for the prediction of the shear resistance of fibre reinforced concrete elements was also evaluated.

Investigating the durability of FRP-masonry elements immersed in water

Nowadays, there is an increasing interest in using fiber reinforced polymers (FRP) for strengthening masonry elements. It has been observed that these materials, when used for externally bonded reinforcement (EBR), improve the performance of masonry components. However, issues such as durability and long-term performance of strengthened elements are still open. The bond between composite material and masonry substrate is a critical mechanism in EBR strengthening techniques, and therefore its durability and long-term performance should be deeply investigated and characterized. In the present study, the influence of water immersion on the bond performance is investigated by performing single-lap shear bond tests on two sets of GFRP-strengthened specimens immersed in water for six months. Different surface preparation techniques are used for each set of specimens to study their effect on the bond degradation. The specimens are prepared following the wet lay-up procedure. The observations and the obtained results are presented and discussed.

Innovative methodologies for the enhancement of the flexural strengthening performance of NSM CFRP technique for RC beams

Tese de Doutoramento em Engenharia Civil

Structure-property relationships in polymer nanocomposites

Tese de Doutoramento em Ciência e Engenharia de Polímeros e Compósitos

A sixth-order finite volume method for the 1D biharmonic operator: application to intramedullary nail simulation

A new very high-order finite volume method to solve problems with harmonic and biharmonic operators for one- dimensional geometries is proposed. The main ingredient is polynomial reconstruction based on local interpolations of mean values providing accurate approximations of the solution up to the sixth-order accuracy. First developed with the harmonic operator, an extension for the biharmonic operator is obtained, which allows designing a very high-order finite volume scheme where the solution is obtained by solving a matrix-free problem. An application in elasticity coupling the two operators is presented. We consider a beam subject to a combination of tensile and bending loads, where the main goal is the stress critical point determination for an intramedullary nail.